<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>

    <style>
        .alpha{
            width: 300px;
            height: 300px;
            background: rgba(0, 0, 0, 0.3);
        }
    </style>
</head>
<body>
    <div class="alpha"></div>
</body>
</html>